Phylogeny and the origin of HIV-1

The origin of human immunodeficiency virus type 1 (HIV-1) is controversial. We show here that viruses obtained from the Democratic Republic of Congo in Africa have a quantitatively different phylogenetic tree structure from those sampled in other parts of the world. This indicates that the structure of HIV-1 phylogenies is the result of epidemiological processes acting within human populations alone, and is not due to multiple cross-species transmission initiated by oral polio vaccination.